1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, and more particularly, it relates to an image forming apparatus including a fixing device having low heat capacity.
2. Related Background Art
In some copying machines and printers, there is provided a fixing device for thermally fixing an image formed on a recording material.
As kinds of the fixing device, there are a heat roller system in which a recording material is pinched and conveyed by a pair of rollers, a film heating system in which a film shifted while being contacted with a heater is used, and induction heating system in which a principle of electromagnetic induction is used. Particularly, since fixing devices of film heating type and of induction heating type have very low heat capacity, heating of the heater can be started after a print signal is inputted and the heating during a waiting condition can be stopped, with the result that such fixing devices save power consumption and are inexpensive.
A most fundamental performance of function required for the fixing device is that a toner image formed on a recording material is thermally fixed without excess and deficiency. This fixing ability depends upon a total amount applied to the toner image being passed through a nip of the fixing device. For example, if a temperature of an environment is high, since temperatures of a pressurizing roller and of the recording material may possibly be high relatively, even when a temperature adjustment temperature during the fixing is relatively low, the total heat amount required for the toner can be obtained. To the contrary, if the environmental temperature is low, since the pressurizing roller and the recording material may possibly be cold, the temperature adjustment temperature during the fixing must be increased to obtain the total heat amount required for the toner.
However, for the purpose of achieving the total heat amount required for the toner, if the temperature adjustment temperature during the fixing is made too high, the toner is heated abruptly, thereby generating a phenomenon (called as xe2x80x9chot offsetxe2x80x9d) in which a portion of the toner image is adhered to the film. If the toner is adhered to the film, endurance of the film may be worsened or the fixing ability may be worsened.
In this way, although the fixing device having low heat capacity in which the heating is started after the print signal is inputted has merits such as low power consumption and cheapness, since the heating is not effected during the waiting condition for waiting the print signal, the pressurizing roller is frequently cold under the low environmental temperature. Accordingly, if the temperature adjustment temperature is not set to be high, the adequate fixing ability cannot be obtained, thereby generating the hot offset.
The present invention is made in consideration of the above circumstances, and an object of the present invention is to provide an image forming apparatus having a fixing device, which can obtain good fixing ability while preventing occurrence of hot offset.
Another object of the present invention is to provide an image forming apparatus having a fixing device, which can prevent hot offset while maintaining low power consumption.
A further object of the present invention is to provide an image forming apparatus comprising image forming means for forming an image on a recording material, a heating member for heating the image on the recording material, a backup roller forming a nip with the heating member, a temperature detecting element for detecting a temperature of an atmosphere, and control means for controlling a power supply to the heating member, wherein when a print signal is inputted, the control means controls power supply (electrical communication) to the heating member in such a manner that the heating member maintains a set temperature in accordance with a detection temperature of the temperature detecting element, and, thereafter, a fixing operation is effected.
A still further object of the present invention is to provide an image forming apparatus comprising image forming means for forming an image on a recording material, a heating member for heating the image on the recording material, a backup roller forming a nip with the heating member, a temperature detecting element for detecting a temperature of an atmosphere, and control means for controlling a power supply to the heating member, and wherein, when a print signal is inputted, the control means controls electrical communication to the heating member in such a manner that the heating member maintains a set temperature by a set time corresponding to a detection temperature of the temperature detecting element, and, thereafter, a fixing operation is effected.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.